Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode configurations).
As briefly discussed above, an external control device can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with the selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by manipulating controls on the external control device to modify the electrical stimulation provided by the neurostimulator system to the patient. However, the number of electrodes available, combined with the ability to generate a variety of complex stimulation pulses, presents a huge selection of stimulation parameter sets to the clinician or patient.
To facilitate such selection, the clinician generally programs the neurostimulator through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neurostimulator to allow the optimum stimulation parameters to be determined based on patient feedback or other means and to subsequently program the neurostimulator with the optimum stimulation parameter set or sets, which will typically be those that stimulate all of the target tissue in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. The computerized programming system may be operated by a clinician attending the patient in several scenarios.
For example, in order to achieve an effective result from SCS, the lead or leads must be placed in a location, such that the electrical stimulation will cause paresthesia. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When electrical leads are implanted within the patient, the computerized programming system, in the context of an operating room (OR) mapping procedure, may be used to instruct the neurostimulator to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient.
Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neurostimulator, with a set of stimulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint the stimulation region or areas correlating to the pain. Such programming ability is particularly advantageous for targeting the tissue during implantation, or after implantation should the leads gradually or unexpectedly move that would otherwise relocate the stimulation energy away from the target site. By reprogramming the neurostimulator (typically by independently varying the stimulation energy on the electrodes), the stimulation region can often be moved back to the effective pain site without having to re-operate on the patient in order to reposition the lead and its electrode array. When adjusting the stimulation region relative to the tissue, it is desirable to make small changes in the proportions of current, so that changes in the spatial recruitment of nerve fibers will be perceived by the patient as being smooth and continuous and to have incremental targeting capability.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes using one or more current-controlled sources for providing stimulation pulses of a specified and known current (i.e., current regulated output pulses), or one or more voltage-controlled sources for providing stimulation pulses of a specified and known voltage (i.e., voltage regulated output pulses).
For example, with reference to FIG. 1, a neurostimulator may have multiple output current sources 1a and multiple current sinks 1b (only one current source 1a and one current sink 1b shown in FIG. 1) that are configured to supply/receive stimulating current to/from the electrodes Ex, Ey, and ultimately to/from tissue (represented by load 5 having a resistance R). The source 1a and sink 1b are sometimes respectively referred to as PDACs and NDACs, reflecting the fact that the source 1a is typically formed of P-type transistors, while the sink 1b is typically formed of N-type transistors. The use of transistors of these polarities is sensible given that the source 1a is biased to a high voltage (V+), where P-type transistors are most logical, while the sink 1b is biased to a low voltage (V−), where N-type transistors are most logical, as shown in FIG. 1. A suitable current generator is disclosed in U.S. Pat. No. 6,181,969 (“the '969 patent”), which is expressly incorporated herein by reference in its entirety.
The output current source 1a and output current sink 1b respectively include current generators 2a, 2b each configured to generate a reference current Iref, and digital-to-analog converter (DAC) circuitry 3a, 3b configured for regulating/amplifying the reference current Iref provided by the current generators 2a, 2b, and delivering output current Iout to the load 5 (having a resistance R). Specifically, the relation between Iout and Iref is determined in accordance with input bits arriving on busses 4a, 4b, which respectively give the output current source 1a and output current sink 1b their digital-to-analog functionality. In accordance with the values of the various M bits on busses 4a, 4b any number of output stages (i.e., transistors M1, M2) are tied together in parallel such that Iout can range from Iref to 2M*Iref.
As shown in FIG. 1 for simplicity, the current source 1a is coupled to an electrode Ex, while the current sink 1b is coupled to a different electrode Ey. However, each electrode may actually be hard-wired to both the current source 1a and the current sink 1b, only one (or neither) of which is activated at a particular time to allow the electrode to selectively be used as either a source or sink (or as neither).
This architecture is shown in FIG. 2, which shows four exemplary electrodes E1, E2, E3, and E4, each having its own dedicated and hard-wired current source 1a and current sink 1b. Thus, an output current source 1a may be associated with electrode E2 (e.g., EX of FIG. 1) at a particular point in time, while an output current sink 1b may be associated with electrode E3 (e.g., EY of FIG. 1) at that time. At a later time, electrodes E2 and E3 could be switched, such that E2 now operates as the sink, while electrode E3 operates as the source, or new sources or sinks could be selected, etc.
Another architecture, shown in FIG. 3, uses a plurality of current sources 1a and sinks 1b, and further uses a low impedance switching matrix 6 that intervenes between the sources/sinks and the electrodes EX. Each source/sink pair is hard-wired together at common nodes 7, such that the switching matrix 6 intervenes between the nodes 7 and the electrodes. Of course, only one of the source or the sink in each pair is activated at one time, and thus the node 7 in any pair will source or sink current at any particular time. Through appropriate control of the switching matrix 6, any of the nodes 7 may be connected to any of the electrodes EX at any time.
Further details discussing various architectures of current source/sink circuitry are provided in U.S. Patent Publication No. 2007/0100399, which is expressly incorporated herein by reference.
Because each current source and current sink requires a relatively large number of switches, it can be appreciated that as the number of current sources and current sinks increases, the complexity and space required to accommodate them increases. It is, thus, desirable to minimize the number of current sources/sinks needed, while still allowing the steering of electrical current between the electrodes.